Traditionally, deflectable medical catheters have been used in interventional procedures to deliver therapies, such as RF energy, or implantables, such as leads or valves, into the body. Medical catheters have also been used for imaging and diagnostic purposes. Finally, medical catheters, such as those with balloons, have been used to modify a patient's anatomy, such as during a structural heart application. In many cases, the aforementioned applications of medical catheters could benefit from the integration of MRI.
MRI has achieved prominence as a diagnostic imaging modality, and increasingly as an interventional imaging modality. The primary benefits of MRI over other imaging modalities, such as X-ray, include superior soft tissue imaging and avoiding patient exposure to ionizing radiation produced by X-rays. MRI's superior soft tissue imaging capabilities have offered great clinical benefit with respect to diagnostic imaging. Similarly, interventional procedures, which have traditionally used X-ray imaging for guidance, stand to benefit greatly from MRI's soft tissue imaging capabilities. In addition, the significant patient exposure to ionizing radiation associated with traditional X-ray guided interventional procedures is eliminated with MRI guidance.
A variety of MRI techniques are being developed as alternatives to X-ray imaging for guiding interventional procedures. For example, as a medical device is advanced through the patient's body during an interventional procedure, its progress may be tracked so that the device can be delivered properly to a target site. Once delivered to the target site, the device and patient tissue may be monitored to improve therapy delivery. Thus, tracking the position of medical devices is useful in interventional procedures. Exemplary interventional procedures include, for example, cardiac electrophysiology procedures including diagnostic procedures for diagnosing arrhythmias and ablation procedures such as atrial fibrillation ablation, ventricular tachycardia ablation, atrial flutter ablation, Wolfe Parkinson White Syndrome ablation, AV node ablation, SVT ablations and the like. Tracking the position of medical devices using MRI is also useful in oncological procedures such as breast, liver and prostate tumor ablations; and urological procedures such as uterine fibroid and enlarged prostate ablations.
MRI uses three fields to image patient anatomy: a large static magnetic field, a time-varying magnetic gradient field, and a radiofrequency (RF) electromagnetic field. The static magnetic field and time-varying magnetic gradient field work in concert to establish both proton alignment with the static magnetic field and also spatially dependent proton spin frequencies (resonant frequencies) within the patient. The RF field, applied at the resonance frequencies, disturbs the initial alignment, such that when the protons relax back to their initial alignment, the RF emitted from the relaxation event may be detected and processed to create an image.
Each of the three fields associated with MRI presents safety risks to patients when a medical device is in close proximity to or in contact either externally or internally with patient tissue. One important safety risk is the heating that may result from an interaction between the RF field of the MRI scanner and the medical device (RF-induced heating), especially medical devices that have elongated conductive structures, such as braiding and pull-wires in catheters and sheaths.
The RF-induced heating safety risk associated with elongated metallic structures in the MRI environment results from a coupling between the RF field and the metallic structure. In this case several heating related conditions exist. One condition exists because the metallic structure electrically contacts tissue. RF currents induced in the metallic structure may be delivered into the tissue, resulting in a high current density in the tissue and associated Joule or Ohmic tissue heating. Also, RF induced currents in the metallic structure may result in increased local specific absorption of RF energy in nearby tissue, thus increasing the tissue's temperature. The foregoing phenomenon is referred to as dielectric heating. Dielectric heating may occur even if the metallic structure does not electrically contact tissue, such metallic braiding used in a deflectable sheath. In addition, RF induced currents in the metallic structure may cause Ohmic heating in the structure, itself, and the resultant heat may transfer to the patient. In such cases, it is important to attempt to both reduce the RF induced current present in the metallic structure and/or eliminate it all together by eliminating the use of metal braid and long metallic pull-wires.
The static field of the MRI will cause magnetically induced displacement torque on any device containing ferromagnetic materials and has the potential to cause unwanted device movement. It is important to construct the catheter shaft and control handle from non-magnetic materials, to eliminate the risk of unwanted device movement.
When performing interventional procedures under MRI guidance, clinical grade image quality must be maintained. Conventional deflectable catheters are not designed for MRI and may cause image artifacts and/or distortion that significantly reduce image quality. Constructing the catheter from non-magnetic materials and eliminating all potentially resonant conductive structures allows the catheter to be used during active MR imaging without impacting image quality. Similarly, it is as important to ensure that the catheter control handle is also constructed from non-magnetic materials thereby eliminating potentially resonant conductive structures that may prevent the control handle being used during active MR imaging.
In many medical procedures in which a catheter is utilized, there is a need for the integration of a sliding component in the distal section of the catheter. Such a sliding component would translate distally and proximally within a distal tip section of the catheter. In conventional sliding distal component assembly designs, translation of the sliding distal component is achieved by pulling or pushing on a stiff rod that is coupled to the sliding distal component. The presence of a stiff rod or cable makes a catheter less flexible, and therefore is not ideal for catheters that are used to navigate tortuous anatomy. In addition, if the catheter or cable is a smaller size, such as 7 Fr or less, there would not be sufficient space in the lumen of the catheter for the stiff rod. This problem necessitates that the rod or cable be smaller than the internal diameter of the catheter. In addition, it necessitates that the rod or cable comprise a metallic composition because metal is the most suitable material for creating a small rod that has acceptable column strength to push the sliding distal component.
For the foregoing reasons, such a long metal rod or cable should not be utilized in those cases in which MRI guidance is employed. Thus, what is needed is an MR compatible sliding distal component mechanism design that could be universally used with both large and small catheters alike.